DESCRIPTION (Principal Investigator's abstract): The neural basis for the generation and control of vertical vestibular-induced eye movements, including adaptive plasticity of the vestibulo-ocular reflex (VOR) will be studied utilizing physiological, morphological, neurochemical, and morphophysiological techniques. The vertical VOR will be visually adapted by employing both miniaturizing and/or magnifying lenses placed in a holder in front of the animals' eyes. Both the normal and adapted VORs will then be utilized as behavioral probes to elucidate the properties of brainstem and cerebellar neurons in vestibulo-ocular pathways. First, we will record extracellularly in alert animals, from Y-group, vestibular nucleus, and cerebellar neurons in normal monkeys and in those whose vertical VOR has been plastically adapted. We will quantify the responses of these cells to ascertain their roles, if any, in the normal VOR. Then, during VOR adaptation, we will ask if there are changes in neuronal firing that parallel the changes in reflex-induced eye movements. We hypothesize that eye movement plasticity is brought about by modified signals transmitted, pre-formed to the extra ocular motor nuclei. Y-group neurons are favorably situated to carry these signals. They are flocculus target neurons that are di- or polysynaptically activated by the VIIIth nerve and they project monosynaptically to the IIIrd and IVth nuclei. Discrete chemical lesions will also be employed in selected nuclear and cerebellar sites to temporarily silence them and then the normal and adapted VORs examined. The synaptic connectivity, morphology, and morphophysiology of neurons will be studied with anatomical tracers and intracellular injection to add information at the cellular and morphological levels that may shed light upon the function of this structure in gaze control. The Marr- Albus-Ito hypotheses and others will be critically tested. The neural basis for the integration of signals related to vertical eye movements will be studied in squirrel monkeys with physiological and morphological techniques. The axons of interstitial nucleus of Cajal neurons will be impaled in alert animals, physiologically characterized in relation to gaze, and injected with tracer to elucidate neuronal structure-function correlations. We will evaluate the organization of the vertical neural integrator to test hypotheses concerning its operation and the necessity for a bilateral structural organization.